Their eyes met across a crowded room. The party chatter ebbed away, and the music slowed. That first lovers' glaze is the staple of the romantic novelist, and scientists believe they have now revealed the true nature of its true attractive power.

According to new research, romance has very little do to with it. That "look" is all about sex and ego.

"It does seem to be a sort of narcissistic thing. People are attracted to people who are attracted to them," said Ben Jones in the Face Research Laboratory at the University of Aberdeen.

"It's really a very basic effect that we are all, at some level at least, aware of - which is that if you smile at people and you maintain eye contact, it makes you more attractive."

He said the work challenges most previous studies of facial attractiveness that have focused on physical characteristics, such as a preference for symmetrical faces or masculine versus feminine features.

"Social signals about how attracted someone else is to you actually seem to be quite important," he said. "You are attracted to people who are attracted to you, and that shows attractiveness is not just about physical beauty."

Dr Jones and his colleagues say they have shown that attraction is based on social cues that say, "I'm interested in you". The most important cue seems to be whether someone is looking directly at you.

The team put together four different sets of digital images - women looking happy, women looking disgusted, men looking happy and men looking disgusted. In each case, the scientists made up pairs of images which were identical except that in one the person was looking directly at the camera and in the other their gaze was averted. Volunteers then rated the relative attractiveness of the images in each pair.

The team found that a direct stare is attractive only if the person giving it looks as if they like you. This preference was even higher if the face in the picture was of the opposite sex.

"What we found at the most basic level is that people like faces with direct gaze more than they like the same faces with averted gaze," said Dr Jones. "In other words, people find it more attractive when they are being looked at."

The results are published in the Proceedings of the Royal Society.

Dr Jones said the results make sense from an evolutionary perspective. "It takes quite a lot of effort to attract a mate and what you want to do is allocate that effort in a more efficient way, in other words in a way that is more likely to help you secure a mate."

So it seems there is no point wasting your time on someone who is just not interested.